1. Field of the Invention
The present invention relates to a misfire detecting system for an internal combustion engine.
2. Description of the Prior Art
As a method for detecting misfire of an internal combustion engine, there has been known a method in which misfire is detected on the basis of a rotational variation of a crankshaft (a variation in the detection cycle for a rotational angle of the crankshaft) in an internal combustion engine.
In some particular operating conditions, however, a variation in the rotation of the crankshaft may occur due to an occasional improper combustion despite a normal operation of the system. There has been an example in which operating conditions are set to forbid the detection of misfire on the ground that the detection of misfire cannot be done accurately (Japanise Patent Application No. 360626/91).
In the above example, as the operating conditions at which the monitor of misfire is forbidden, there are set a condition in which the engine cooling water temperature or the intake air temperature is below a predeterminded value, a fuel cut condition and a lean running condition.
For example, however, during vehicular running at a low load such as driving on a downhill at a highland, an amount of fuel to be injected which is too small to be controlled, is required and actually there sometimes occurs a case where fuel is not injected and the air fuel ratio becomes a lean side, thus causing improper combustion. Heretofore, even in such a case, the detection of misfire has been conducted, sometimes resulting in the occurrence of misdetection.